Process for preparing a flame retardant polyamide compound

ABSTRACT

The invention relates to a process for preparing a flame retardant polyamide compound comprising melt-mixing of a composition comprising at least a polyamide polymer having a weight-average molecular weight of at least 10,000 g/mol, an amount of 1-100 wt. % relative to the total weight of polyamide of a flame retardant, and an amount of 0.1-30 wt. %, relative to the total weight of polyamide, of a polyamide oligomer having a weight-average molecular weight of at most 7,500 g/mol.

The present invention relates to a process for preparing a flameretardant polyamide compound comprising melt-mixing of a compositioncomprising at least a polyamide polymer having a weight-averagemolecular weight of at least 10,000 g/mol and a flame retardant.

Such a process is known from EP-0794976-B1. This patent describes flameretardant polyamide compounds comprising different components includinga polyamide and a flame retardant, which compounds can be prepared by aprocess wherein the components are dry-blended and consequently fed intoa melt-mixing apparatus, for instance an extruder. As the flameretardants organic halogen-free flame retardants are mentioned, moreparticular the triazine compounds melamine and melamine cyanurate.Melamine and melamine cyanurate have found widespread application inflame retardant plastic compounds. These compounds have the practicaladvantages that already at relatively low concentration adequate flameretardancy can be obtained without the use of synergists, nodiscoloration of the compound occurs under normal conditions and nocorrosion of the processing apparatus is observed. In the cited patentit is reported that the flame retarding effect of triazine compounds isthought to originate from the decomposition of these compounds atheating into nitrogen containing gaseous compounds that act as flameextinguishers.

In the same patent, it is mentioned that melamine, and to a lesserextent melamine cyanurate, have the disadvantage that during processingmelamine is deposited in the mould, which may cause obstruction ofventilation ducts and makes regular stops of the production processnecessary to clean the mould. Although melamine cyanurate shows thisdisadvantageous phenomenon of mould deposit to a lesser extend thanmelamine, its use is further limited due to the fact that at highertemperatures, for instance at temperatures higher than 270° C.,polyamide melts containing melamine cyanurate are unstable and show adecrease in melt viscosity. At temperatures of 290° C. and higher inmany cases gas evolution is so high that processing of compounds withmelamine cyanurate is practically impossible.

This problem of limited stability at elevated temperature is also metwith other halogen-free flame-retardants, such as melaminepolyphosphates. The limited temperature stability of the halogen-freeflame retardants raises problems in the preparation of flame retardantpolyamide compounds in the class of high temperature engineeringpolyamides via a melt-mixing process, resulting in degradation of theflame retardant and in polyamide compounds having a lower flameretardancy than could be expected on the basis of the content of flameretardant in the compounds. These phenomena are in particular observedwith glass fiber reinforced polyamides.

Therefore, in high temperature engineering polyamides, for instancepolyamide-6,6, (melting temperature 265° C.), polyamide-4,6, (meltingtemperature 290° C.), semi-aromatic polyamides, with even higher meltingtemperatures, and the copolyamides thereof (for instancePA-6,6/6,T/6,I), melamine and melamine cyanurate cannot be used and onehas to rely on the halogenated organic compounds, for instancebrominated polystyrene, to impart flame retardant properties to thesepolyamides. However, these halogenated flame retardant compounds arealso heat sensitive in the sense that compounds with these flameretardants usually are discolored or give rise to corrosion ofprocessing equipment. These problems are further emphasized in glassfiber reinforced compounds based on high temperature engineeringpolyamides and halogenated flame retardant.

As a solution to these problems, EP-0794976-B1 claims polyamidecompounds comprising melam as the flame retardant and the possiblepreparation of these compounds via a melt-mixing process.

The object of the present invention is to provide a melt-mixing process,which allows preparation of a flame retardant polyamide compoundcomprising a heat sensitive halogen-free flame retardant and/or ahalogenated flame retardant in combination with high temperatureengineering polyamides as the polyamide, which compound shows improvedflame retardancy properties and/or improved color compared to the knowncompounds.

This object is achieved with the process wherein the composition that ismelt-mixed comprises an amount of 0.1-30 wt. %, relative to the totalweight of polyamide, of a polyamide oligomer having a weight-averagemolecular weight of at most 7,500 g/mol, with the proviso that thecomposition that is melt-mixed does not consist of:

-   i) 100 parts by weight of a polyamide polymer, 0.001-10 parts by    weight of a polyamide oligomer with a molecular weight of 5000 or    less and having hydrocarbon radicals with 5-30 carbons as the    terminals, and 1-25 parts by weight of a triazine flame retardant;-   ii) 100 parts by weight of a polyamide-6,6 polymer, 15 parts by    weight of a polyamide oligomer with a molecular weight of 1000 and    consisting of the condensation product of stearic acid,    ethylenediamine, and sebacic acid, and 7 parts by weight of melamine    cyanuric acid; or-   iii) 100 parts by weight of a polyamide-6 polymer, 0.5 parts by    weight of polyamide oligomer with a molecular weight of 1200 and    consisting of the condensation product of stearyl amine,    ethylenediamine, and sebacic acid, and 27 parts by weight of    melamine cyanuric acid.

With the process according to the invention, using an organichalogen-free flame retardant as the flame retardant, polyamide compoundswith a lower total burning time in flame retardancy tests, compared tothe compounds per the prior art, can be prepared. The compounds alsoshow, as an additional advantage, improved mechanical properties. Withthe process according to the invention, wherein the flame retardant is ahalogenated organic compound, a polyamide compound with reduceddiscoloration can be prepared. Furthermore, these compounds show a goodstrand quality upon extrusion of the melt after mixing, even when theprocess is carried out at higher screw speed.

JP-5214246-A (Toray, 1992) describes a polyamide compound wherein0.001-10 parts by weight of a polyamide oligomer with a molecular weightof 5000 or less and modified with hydrocarbon radicals with 5-30 carbonatoms as terminals groups, and 1-25 parts by weight of a triazine flameretardant have been melt-mixed with 100 parts by weight of a polyamidepolymer. With this compound, the Japanese patent application aims at asolution for the problem of flame resistance decline, which occurs byaddition of a mould-separating agent. The described polyamide compoundis claimed to have flame resistant and mould release properties.JP-5214246-A does not relate to the problem of, or a solution for,instability of flame retardants at elevated temperatures occurringduring melt-mixing of a composition comprising at least a polyamide anda flame retardant. More particularly, to avoid similar problems duringmolding, JP-5214246-A teaches to limit the amount of flame retardant to25 weight parts, more preferably 20 weight parts and even morepreferably 15 weight parts, relative to 100 weight parts polyamidepolymer, because when the rate of flame retardant exceeds the higherlimit gas is produced at the time of moulding, and the outer appearanceof the moulded product is damaged and mechanical properties decline.JP-5214246-A also teaches to limit the amount of polyamide oligomer to10 weight parts, because when the rate of polyamide oligomer exceeds 10weight parts, gas is produced at the time of moulding, and the outerappearance of the moulded product is damaged and mechanical propertiesdecline.

JP-5214246-A furthermore reports, as comparative examples, melt-mixingof a composition consisting of 100 parts JP-5214246-A by weight of apolyamide-6,6 polymer, 15 parts by weight of a polyamide oligomer with amolecular weight of 1000 and consisting of the condensation product ofstearic acid, ethylenediamine, and sebacic acid, and 7 parts by weightof melamine cyanuric acid; and a composition consisting of 100 parts byweight of a polyamide-6 polymer, 0.5 parts by weight of polyamideoligomer with a molecular weight of 1200 and consisting of thecondensation product of stearyl amine, ethylenediamine, and sebacicacid, and 27 parts by weight melamine cyanuric acid. JP-5214246-A doesnot report the advantageous features of our invention for these twocomparative examples.

The compositions which are melt-mixed in the claimed process accordingto JP-5214246-A, as well as the above two comparative examples mentionedtherein, accidentally fall in the range of the invention and aredisclaimed.

With the process according to the invention, compositions comprising apolyamide polymer, a flame retardant and an oligomer with a molecularweight of 5000 or less and modified with hydrocarbon radicals with 5-30carbon atoms as terminals groups can be melt-mixed and still givingproducts with a good outer appearance and/or with improved mechanicalproperties when the amount of flame retardant is higher than 25 parts byweight and/or the amount of polyamide oligomer is higher than 10 partsby weight, the amounts relative to 100 parts by weight of the polyamidepolymer. The same good results are obtained when the molecular weight ofthe polyamide oligomer is above 5000.

In the context of the present invention, a polyamide compound isunderstood to be a polyamide composition that is obtainable bymelt-mixing of the constituting components of the polyamide compositionand is suitable for being used in a moulding process for preparing ashaped article. The polyamide compound may have different forms, forexample, but not limited thereto, a melt (for example in an apparatussuitable for preparing moulded parts), extruded strands, choppedgranules, and molded parts (for example when the polyamide compositionafter being melt-mixed is directly shaped into a moulded part).

With a polyamide composition is understood the total of ingredients orconstituents, which can be used for preparing a polyamide compound. Thepolyamide composition may have the form of a dry blend. The compositionmay also be formed by separately adding the respective constituents orcombinations thereof, to an apparatus suitable for preparing saidpolyamide compound.

In the context of the present invention a polyamide polymer isunderstood to be a high-molecular polyamide with a weight-averagemolecular weight of at least 10,000 g/mol, preferably at least 15,000g/mol, more preferably at least 20,000 g/mol.

The polyamide oligomer is herein understood to be a low-molecularpolyamide with a weight-average molecular weight of at most 7500.Preferably, the weight-average molecular weight is lower than the“molecular weight between entanglements” of the high-molecular polymer.This “molecular weight between entanglements” is for example 5,000 g/molin the case of PA-6. Also preferably the weight average molecular weightof the polyamide oligomer is at most 5,000 g/mol, more preferably atmost 4,000 g/mol, even more preferably at most 3,000 g/mol. Themolecular weight of the polyamide oligomer may not be too low either, toavoid the risk of for example the glass transition temperature beinglowered. Preferably the weight-average molecular weight is greater thanapproximately 1,000 g/mol.

Suitable polyamides, for both the polyamide polymer and the polyamideoligomer, are all the polyamides known to a person skilled in the art,comprising crystalline, semi-crystalline and amorphous polyamides, thatare melt-processable. Examples of suitable polyamides according to theinvention are aliphatic polyamides, for example PA-6, PA-11, PA-12,PA4,6, PA-4,8, PA-4,10, PA-4,12, PA-6,6, PA-6,9, PA-6,10, PA-6,12,PA-10,10, PA-12,12, PA-6/6,6-copolyamide, PA-6/12-copolyamide,PA-6/11-copolyamide, PA-6,6/11-copolyamide, PA-6,6/12-copolyamide,PA-6/6, 10-copolyamide, PA-6,6/6, 10-copolyamide, PA-4,6/6-copolyamide,PA-6/6,6/6, 10-terpolyamide, and copolyamides obtained from1,4-cyclohexanedicarboxylic acid and 2,2,4- and2,4,4-trimethylhexamethylenediamine, aromatic polyamides, for examplePA-6,I, PA-6,I/6,6-copolyamide, PA-6,T, PA-6,T/6-copolyamide,PA-6,T/6,6-copolyamide, PA-6,I/6,T-copolyamide,PA-6,6/6,T/6,I-copolyamide, PA-6,T/2-MPMDT-copolyamide(MPMDT=2-methylpentamethylene diamine), PA-9,T, copolyamides obtainedfrom terephthalic acid, 2,2,4- and 2,4,4-trimethylhexamethylenediamine,copolyamide obtained from isophthalic acid, laurinlactam and3,5-dimethyl-4,4-diamino-dicyclohexylmethane, copolyamides obtained fromisophthalic acid, azelaic acid and/or sebacic acid and4,4-diaminodicyclohexylmethane, copolyamides obtained from caprolactam,isophthalic acid and/or terephthalic acid and4,4-diaminodicyclohexyl-methane, copolyamides obtained from caprolactam,isophthalic acid and/or terephthalic acid and isophoronediamine,copolyamides obtained from isophthalic acid and/or terephthalic acidand/or other aromatic or aliphatic dicarboxylic acids, optionallyalkyl—substituted hexamethylenediamine and alkyl-substituted4,4-diaminodicyclohexylamine, and also copolyamides and mixtures of theaforementioned polyamides.

Preferably polyamides are chosen from the group comprising PA-6, PA-6,6,PA-6,10, PA-4,6, PA-11, PA-12, PA-12,12, PA-6,I, PA-6,T,PA-6,T/6,6-copolyamide, PA-6,T/6-copolyamide, PA-6/6,6-copolyamide,PA-6,6/6,T/6,I-copolyamide, PA-6,T/2-MPMDT-copolyamide, PA-9,T,PA-4,6/6-copolyamide and mixtures and copolyamides of the aforementionedpolyamides. More preferably PA-6,I, PA-6,T, PA-6,6, PA-6,6/6T,PA-6,6/6,T/6,I-copolyamide, PA-6,T/2-MPMDT-copolyamide, PA-9,T orPA-4,6, or a mixture or copolyamide thereof, is chosen as the polyamide.

The low molecular weight polyamide oligomer may be chosen to have thesame composition as the high molecular weight polyamide; the twopolyamides, i.e. the oligomer and the polymer, may also be chosen tohave different compositions.

The high molecular weight polyamide polymer in the process according tothe invention may optionally contain modified end groups, for exampleamine end groups modified with a mono-carbooxylic acid and/or carboxylicacid end groups modified with a mono-functional amine. Modifiedend-groups in the high molecular weight polyamide polymer mayadvantageously be applied for improved melt-stability of the compositionduring preparation of the compound by melt-mixing and for improvedmelt-stability of the compound during moulding of said compound forpreparing a moulded part.

Preferably, the polyamide polymer is a high temperature engineeringpolyamide, defined by a melting temperature of at least 260° C. Morepreferable, the melting temperature is at least 270° C., even morepreferable at least 280° C. and most preferably at least 290° C. Thehigher the melting temperature of the polyamide polymer, the morepronounced the effect of addition of the polyamide oligomer in themelt-mixing process is in the properties of the compound formed.

The polyamide oligomer preferably has a melting temperature of at most20° C. above, more preferable at most 10° C. above, and even morepreferable at most equal to the melting temperature of the polyamidepolymer. The advantage of a polyamide oligomer with a meltingtemperature that does not exceed the melting temperature of thepolyamide polymer too much or even better with a melting temperaturethat does not exceed the melting temperature of the polyamide polymer atall, is that the processing temperature necessary for the melt-mixingprocess can be kept as low as or can even be reduced compared to theprocess without polyamide oligomer and further reduces the effects ofdegradation of the flame retardant.

The polyamide oligomer preferably has a melting temperature of at least260° C., more preferable at least 270° C., even more preferable at least280° C. The advantage of a polyamide oligomer with a higher meltingtemperature, in particular when used in a larger amount, is that thehigh temperature mechanical properties of the polyamide compound arebetter retained or even improved.

A polyamide that is eminently suited for the polyamide oligomer in theprocess according to the invention is polyamide-4,6. Polyamide-4,6oligomer has a melting temperature of about 290° C. The advantage ofpolyamide-4,6 oligomer is that it can be produced on industrial scaleand that it can be combined with a wide range of high temperatureengineering polyamides.

In the context of this application “PA-4,6” is understood to be apolyamide of which at least 50%, preferably at least 75%, morepreferably at least 90%, consists of tetramethyleneadipamide units.PA-4,6 can be prepared through polycondensation of tetramethylenediamineand adipic acid or an adduct thereof, optionally in the presence ofother polyamide-forming monomers, for example ε-caprolactam, a differentdiamine, for example hexamethylenediamine or a different carboxylicacid, for example isophthalic acid or cyclohexanedicarboxylic acid.

The polyamide oligomer in the process according to the invention mayoptionally contain modified end groups, for example amine end groupsmodified with a mono-functional carboxylic acid and/or carboxylic acidend groups modified with a mono-functional amine. Mono-functionalcarboxylic acids and/or mono-functional amines may advantageously beapplied during the preparation of the polyamide oligomer as a chainstopper to control the molecular weight of the polyamide oligomer.Preferably, the polyamide oligomer comprises at most 50%, morepreferably at most 25%, modified end groups relative to the total numberof end groups. The advantage of a polyamide oligomer with at most 25%modified end groups in the process according to the invention is thatthe mechanical properties of the resulting compound, when comprising ahalogen free flame retardant, are further improved.

The composition that is melt-mixed in the process according to theinvention comprises 0.5-30 wt. % oligomer, relative to the total amountof polyamide. A person skilled in the art can choose the amount ofoligomer within the given range, depending on the desired properties ofthe compound. Larger amounts of the oligomer can be used and may evenresult in a better performance of the flame retardant, however, toolarge amounts may adversely affect the mechanical properties. Preferablyan amount of 1-20 wt. % oligomer, relative to the total amount ofpolyamide, more preferably 2-15 wt. % oligomer is chosen.

Suitable flame retardants that can be used in the process according tothe invention, are halogen-free flame retardants as well as halogencontaining flame retardants.

In a preferred mode of the process according to the invention, thecomposition that is melt-mixed comprises a halogen-free flame retardantas the flame retardant. With this process polyamide compounds with lowertotal burning times in flame retardancy tests can be prepared than withthe same flame retardants in a process without a polyamide oligomer. Theresulting compounds also show, as an additional advantage, improvedmechanical properties.

Suitable halogen-free flame retardants are:

-   -   metal-containing compounds such as magnesium hydroxide and        aluminium hydroxide;    -   nitrogen-containing compounds such as guanamine based compounds        and melamine based compounds;    -   nitrogen and phosphorus containing compounds such as ammonium        polyphosphate and melamine based phosphorous compounds.    -   phosphorus-containing compounds such as red phosphorus,        phosphazene-based compounds and organic phosphorus compounds

Suitable melamine based compounds that can be chosen as the nitrogencontaining compound are for example melamine, melamine derivatives,melamine condensation products and mixtures thereof. In the context ofthis application a “melamine derivative” is understood to be melaminewith one or more amine groups having been substituted with one or morealkyl, aryl, aralkyl or cycloalkyl groups, for example to be chosen fromthe group comprising methyl, ethyl, ethenyl, phenyl or toluyl. Examplesof such melamine derivatives are N,N′,N″-triphenylmelamine. Anotherexample of a melamine derivative is melamine cyanurate. In the contextof this application a “melamine condensation product” is understood tobe a compound in which two or more melamine compounds are connected toone another, for example melam, melem, melon and higher oligomers andmenthone, which condensation products can for example be obtained usingthe process described in WO 96/16948.

Preferably, the nitrogen-containing compound is melamine, melaminecyanurate, melam, melem and melon or mixtures thereof are chosen. Theadvantage is that further processing of the polyamide compounds iseasier and that deposition of volatile components in the mould isreduced.

Suitable melamine-phosphorus compounds that can be used as the flameretardant in the present invention are, for example, melaminephosphates, like melamine phosphate, melamine pyrophosphate and melaminepolyphosphate, for example Melapur® 200 (DSM, the Netherlands) andPMP-100® (Nissan Chemical Industries, Japan), and phosphates of melaminecondensation products, like melam polyphosphate, for example PMP-200®(Nissan Chemical Industries), and melem polyphosphate, for examplePMP-300® (Nissan Chemical Industries),

Suitable organic phosphorus compounds are for example organicphosphates, phosphites, phosphonates, phosphinates and phosphine oxides.

Preferably, phosphates, phosphonates or phosphinates are chosen.Examples of such phosphorus compounds are described in for example the“Encyclopedia of Chemical Technology”, Vol. 10, p. 396 ff. (1980). Manyare commercially available, for exampleresorcinol-bis(diphenylphosphate) oligomer, under the trade nameFyrolflex® RDP from AKZO-Nobel, NL; cresyl-diphenylphosphate (CDP) underthe trade name Kronitex® CDP from FMC, UK; trimethylolpropanol ester ofmethyl phosphoric acid, under the trade name Amgard P45 from Albrightand Wilson, USA; trimethylolpropanol ester of methylphosphonic acid,under the trade name Antiblaze® 1045 from Albright & Wilson, USA;polypentaerythritol phosphonate under the trade name Cyagard® RF 1041from American Cyanamid, USA; Hostaflam® OP 910, a mixture of cyclic di-and triphosphonates from Clariant, Germany.

Preferably, a phosphorus compound with a low volatility is chosen as theflame retardant. It is also advantageous to use an organic phosphoruscompound with a phosphorus content of at least 14 wt. %, preferably atleast 18 wt. %. Examples of such organic phosphorus compounds with aphosphorus content of at least 14 weight % are Amgard-P45 and the metalphosphinates as for instance in U.S. Pat. No. 4,208,321 and U.S. Pat.No. 3,594,347.

In another preferred mode of the invention, the process is carried outwith a halogenated organic compound as the flame retardant is. With thisprocess, polyamide compounds with reduced discoloration can be preparedcompared to the process with the same flame retardants and without apolyamide oligomer. Furthermore, the resulting compounds show a goodstrand quality upon extrusion of the melt after mixing, even when theprocess is carried out at higher screw speed.

Suitable for use as the halogen-containing system are for examplesystems that contain halogen-containing flame retardants as the flameretardant, for example brominated polystyrene, for example Pyrochek®68PB and Saytex® HP7010, both from Albemarle (USA), brominatedpolyphenylene ether, for example PO64P® from Great Lakes (USA),polydibromostyrene, for example PDBS80® from Great Lakes,polytribromostyrene, polypentabromostyrene, polydichlorostyrene,polytrichlorostyrene, polypentachlorostyrene,polytribromo-alpha-methylstyrene, polydibromo-p-phenylene oxide,polytribromo-p-phenylene oxide, polydichloro-p-phenylene oxide,polybromo-p-phenylene oxide, polybromo-o-phenylene oxide,pentabromobenzyl acrylate, for example FR1025® from AmeriBrom (USA),ethylene bis-tetrabromo-phtalimide, for example Saytex® BT-93W fromAlbemarle (USA), polybromobiphenyl, brominated phenoxy- andchlorine-containing flame retardants such as DeChlorane® (OccidentalChemical Corporation, USA) and other brominated compounds such asSaytexe 8010 from Albemarle (USA).

In the process according to the invention, also combinations ofdifferent flame retardants may be used. Suitable combinations are forexample combinations of nitrogen-containing flame retardants andphosphorous containing flame retardants, like melamine condensationproducts combined with phosphorous compounds chosen from the groupconsisting of organic phosphates, phosphates, phosphonates andphosphinates. Also combinations of halogen-free and halogen containingflame retardants may be used. A person skilled in the art will be ableto select the optimal combination for his own purposes.

In the flame-retardant compounds prepared by the process according tothe invention the flame retardant is used in an amount between 1 and 100parts by weight, relative to a total amount of polyamide of 100 parts byweight. Preferably, the amount is at least 10 parts by weight, morepreferably at least 20 parts by weight and most preferably at least 30parts by weight, relative to the total amount of polyamide of 100 partsby weight. A higher minimum amount of flame retardant is advantageouslyapplied in compounds having a higher flame retardancy performance.

Also preferably, the amount of flame retardant is at most 90 parts byweight, more preferably at most 85 parts by weight and most preferablyat most 80 parts by weight, relative to the total amount of polyamide of100 parts by weight. A lower minimum amount of flame retardant isadvantageously applied in compounds having an increased toughness andimpact resistance. The optimal amount can in principle be determinedexperimentally by a person skilled in the art of formulating polyamidecompounds through systematic research.

In the process according to the invention, next to the flame retardantalso a synergist may be used. Suitable synergists are:

-   -   antimony-containing compounds, for example antimony trioxide,        for example Bluestar® RG (Campine, Belgium), antimony        tetraoxide, antimony pentoxide, potassium antimonite, sodium        antimonate, for example Pyrobloc® SAP-2 (Cookson Specialty        Additives), antimony tartrate;    -   Group IIA and IIB metal borates, like zinc borate, for example        Firebrake® ZB (Borax Inc., USA)    -   magnesium hydroxide, aluminium hydroxide, iron oxide, zinc        oxide, calcium oxide and analogous substances.

In particular, the halogen-containing flame retardants are typicallyused in combination with an antimony-containing compound.

Other substances that promote the flame retardancy may optionally alsobe added, for example carbon-forming substances such as polyphenyleneether and polycarbonate and substances modifying the dripping behaviour,for example fluoropolymers such as polytetrafluoroethylene.

Also components may be added which function as an anti-drip components.Suitable anti-drip components are, for example, polymers prepared frommonomers comprising at least an ethylenically unsaturated monomer and atleast a carboxylic acid containing monomer. Preferably, the monomerscomprise ethylene as the ethylenically unsaturated monomer.

The composition melt-mixed with the process according to the inventionmay also contain other additives known to a person skilled in the artthat are customarily used in polymer compounds, providing they do notessentially detract from the invention, in particular fillers, pigments,processing aids, for example mould release agents, agents acceleratingcrystallization, nucleating agents, lubricants, softeners, UV and heatstabilizers and the like. In particular, the composition according tothe invention contains an inorganic filler or reinforcing agent.Suitable for use as an inorganic filler or reinforcing agent are all thefillers known to a person skilled in the art. Suitable reinforcingagents are, for example glass fibres, metal fibres, graphite fibres,aramide fibres, glass beads, aluminium silicates, asbestos, mica, clay,calcined clay and talcum.

The process according to the invention is in particular advantageouslyapplied for preparing a flame retardant polyamide compound comprising areinforcing agent. Preparation of a flame retardant polyamide compoundcomprising a reinforcing agent generally suffers even more fromdeterioration of the flame retardant, or the negative effects thereofupon the properties of the polyamide compound, than non-reinforcedcompounds. With the process according to the invention flame retardantreinforced polyamide compounds can be prepared that show a substantiallyreduced negative effect of the flame retardant on the mechanicalproperties of the compound.

Preferably, glass fibres are chosen as the reinforcing agent. With theprocess according to the invention, also compounds with higher glasscontent, without a noticeable effect on the flame retardant and withoutsignificant decline in flame retardant properties, can be prepared.

The process according to the invention can be carried out in amelt-mixing apparatus, for which any melt-mixing apparatus known to theman skilled in the art of preparing polymer compounds by melt-mixing canbe used. Suitable melt-mixing apparatus are, for example, kneaders,Banburry mixers, single screw extruders and double screw extruders.Melt-mixing is typically performed at a processing temperature, which isabove the melting temperature of the high molecular weight polyamide,thereby forming a polymer melt. In the process according to theinvention for the preparation of a polyamide compound, comprisingpolyamide polymer, polyamide oligomer and flame retardant asconstituting components amongst possible other constituting components,the constituting components are fed to a melt-mixing apparatus andmelt-mixed in that apparatus. The constituting components may be fedsimultaneously as a powder mixture or granule mixer, also known-asdry-blend, or may be fed separately. If fed separately, preferably thepolyamide components, i.e. the polymer and the oligomer, are first fedand melted, and than the flame retardant is added to and mixed with thepolymer melt. This has the advantage that the exposure of the flameretardant to high temperature and high shear is limited and lessdeterioration occurs, resulting in further improved flame retardancy orcolour retention. Polyamide polymer and polyamide oligomer may also befed separately.

The invention also relates to a flame retardant polyamide compoundcomprising a polyamide polymer having a weight-average molecular weightof at least 10,000 g/mol and a flame retardant, wherein the compoundcomprises an amount of 0.1-30 wt. %, relative to the total weight ofpolyamide, of a polyamide derived from a polyamide oligomer with amolecular weight below 7500 g/mol, with the proviso that the compounddoes not consist of:

-   i) 100 parts by weight of a polyamide polymer, 0.001-10 parts by    weight of a polyamide oligomer with a molecular weight of 5000 or    less and having hydrocarbon radicals with 5-30 carbons as the    terminals, and 1-25 parts by weight of a triazine flame retardant;-   ii) 100 parts by weight of a polyamide-6,6 polymer, 15 parts by    weight of a polyamide oligomer with a molecular weight of 1000 and    consisting of the condensation product of stearic acid,    ethylenediamine, and sebacic acid, and 7 parts by weight of melamine    cyanuric acid; or-   iii) 100 parts by weight of a polyamide-6 polymer, 0.5 parts by    weight of a polyamide oligomer with a molecular weight of 1200 and    consisting of the condensation product of stearyl amine,    ethylenediamine, and sebacic acid, and 27 parts by weight of    melamine cyanuric acid.

The polyamide compound according to the invention has the advantages ofthe products obtained by the inventive process reported above.

Typically, the polyamide compound according to the invention consistsof:

-   a) 70-99.9 parts by weight of a polyamide polymer having a    weight-average molecular weight of at least 10,000 g/mol-   b) 0.1-30 parts by weight of a polyamide oligomer having a molecular    weight of at most 7500 g/mol, whereby the total amount of a)+b) is    100 parts by weight-   c) 1-100 parts by weight of a flame retardant-   d) 0-50 parts by weight of a reinforcing agent-   e) 0-25 parts by weight of at least one other component.

Preferred embodiments of the inventive polyamide compound directlyrelate to the preferred embodiments of the process according to theinvention described above.

The invention also relates to the use of a flame retardant polyamidecompound according to the invention for preparing a molded part. Theadvantage is that processing of the compound can be done at lowerprocessing temperatures and/or lower pressure than correspondingcompounds without a polyamide oligomer. Furthermore, parts prepared fromcompounds comprising an organic halogen-free flame retardant, have abetter flame retardancy and improved mechanical properties, whereasparts comprising a halogenated organic compound as the flame retardantshow reduced discoloration.

The invention will now be elucidated with reference to the followingexamples, but is not limited thereto. Materials PP-A Polyamide polymer:Stanyl ® KS200 (ex DSM, The Netherlands): Polyamide-4,6 polymer, Mw =36000, viscosity number (formic acid) = 160; T_(melt) = 295° C. PO-APolyamide oligomer: (ex DSM, The Netherlands): M_(w) = 2,000, T_(melt) =288° C. MPP-200 Melamine polyphosphate: Melapur 200 (ex DSM, TheNetherlands): nitrogen content 42-44 wt. %; phosphor content 12-14 wt.%. PMP-100 Melamine polyphosphate; (Nissan Chemical Industries Ltd);phosphor content 14.5 wt. %. Glass fibres Standard glass fibres forpolyamide compounds; average fiber diameter 10 μm. FR-BR Brominatedpolystyrene: Pyrocheck 68PBC (ex Albemarle); Br content 68 wt. %.Sb2O3-MB Antimone trioxide: Antiox GR 2617 (ex Campine); 80% masterbatchin polyamide-6.Determination of the Physical Properties

-   Viscosity number: determined in acetic acid, according to ISO 307.-   Tensile strength: determined at 23° C. and 5 mm/min, according to    ISO 527-   Elongation at break: determined at 23° C. and 5 mm/min, according to    ISO 527-   Notched Izod: determined at 23° C. according to ISO 180/1A-   Molecular weight: determined with the aid of standard GPC techniques-   Melting point: determined with the aid of DSC (2nd run, 10°    C./min.).-   Flame retardancy: determined according Underwriters Laboratories    test method UL 94, using 0.8 mm test bars, conditioned for 48 hours    at 23° C., 50% relative humidity, respectively for 168 hours at 70°    C.-   Bulk density: determined according ASTM D 1895-96 Test Method A.    Preparation of Polyamide Compounds

EXAMPLE I AND COMPARATIVE EXPERIMENT A

Polyamide compounds according Example I, comprising a combination ofPP-A and PO-A, and Comparative Experiment A, only comprising PP-A as thepolyamide, both comprising 30 weight % of MPP-200, relative to the totalweight of the compound (see Table I) were prepared by melt-mixing theconstituent components on a Werner & Pfleiderer ZSK-40 twin screwextruder using a 300° C. flat temperature profile. The constituents werefed via a hopper, glass fibers were added via a side feed. Throughputwas 60 kg/h and screw speed was 250 rpm. The polymer melt was degassedat the end of the extruder. The melt was extruded into strands, cooledand chopped into granules.

The granules were injection moulded into test bars according to ISO527/1A multipurpose specimens and UL 94 test bars of 0.8 mm thick. Thetest bars were used to measure the flame retardant properties andmechanical properties of the compounds, the results of which have beenreported in Table I. TABLE I Composition and results for Example I andComparative Experiment A Comparative Experiment A Example IComponents(weight %): PP-A 40 34 PO-A — 6 MPP-200 30 30 Glass fibre 3030 Throughput in kg/h 60 60 Strand formation Very bad o.k. Notched Izod(kJ/m²) 8.6 9.0 Tensile strength (MPa) 101 110 Elongation at break (%)0.8 1.0 UL 94 V flammability properties: 0.8 mm, 70° C./168 h V0-Classification: 100% 100% Total burning time 23 21 0.8 mm, 23° C./50%RH/48 h V0-Classification 100% 100% Total burning time 19 10

EXAMPLE II AND COMPARATIVE EXPERIMENT B

Example II and Comparative Experiment B represent compounds similar toExample I and Comparative Experiment A, except that PMP-100 was used asthe flame retardant. The processing conditions, applied for thepreparation of the compounds, were the same as for Example I andComparative Experiment. The test results obtained from injection-moldedparts prepared from respective granulates have been collected in TableII. TABLE II Composition and results for Example II and ComparativeExperiment B Comparative Experiment B Example II Components(weight %):PP-A 40 34 PO-A — 6 PMP-100 30 30 Glass fibre 30 30 Throughput (kg/h) 6060 Strand formation o.k. o.k. Tensile strength (MPa) 109 134 Elongationat break (%) 1.0 1.3 UL 94 V flammability properties: 0.8 mm, 70° C./168h V0- classification: 80% 100% Total burning time (sec) 47 34 0.8 mm,23° C./50% RH/48 h V0-classification  0%  60% Total burning time (sec)90 45

EXAMPLES III-VIII AND COMPARATIVE EXPERIMENT C-D

Examples III-VIII and Comparative Experiment C-D represent polyamidecompounds with a halogenated flame retardant system based on FR-68 andSb2O3, further comprising regular glass fibres, stabilizers and alubricant as additives. Comparative Experiments C-D comprise only PP-Aas the polyamide, whereas Examples III-VIII comprise a combination ofPP-A and PO-A, with different levels of PO-A. Furthermore, the compoundswere prepared on a Berstdorf ZE-40 twin-screw extruder at similartemperature settings (flat profile 300° C.) and 175 kg/h throughput andat two levels of screw speed. The quality of the resulting extrudateswas measured in terms of strand quality, colour and bulk density of thepellets. For the respective compounds, process conditions and resultssee Table III. TABLE IV Components (weight %): CE-C CD-D Ex-III Ex-IVEx-V Ex-VI Ex-VII PP-A 42.74 42.74 38.74 38.74 40.74 40.74 41.74 PO-A —— 4 4 2 2 1 FR-BR 18.75 18.75 18.75 18.75 18.75 18.75 18.75 Sb2O3 7.817.81 7.81 7.81 7.81 7.81 7.81 Additives 0.7 0.7 0.7 0.7 0.7 0.7 0.7Glass fibre 30 30 30 30 30 30 30 Screw speed (rpm) 300 600 300 600 300600 600 Strand quality o.k. Irregular, o.k. o.k. o.k. o.k. o.k. swollenStrand colour Beige Dark Beige Beige Beige Beige Beige beige/grey Bulkdensity 0.67 0.40 0.67 0.67 0.71 0.65 0.5 (g/cm3)

1. Process for preparing a flame retardant polyamide compound comprisingmelt-mixing of a composition comprising at least a polyamide polymerhaving a weight-average molecular weight of at least 10,000 g/mol and aflame retardant, characterized in that the composition comprises anamount of 0.1-30 wt %, relative to the total weight of polyamide, of apolyamide oligomer having a weight-average molecular weight of at most7.500 g/mol, with the proviso that the composition that is melt-mixeddoes not consist of: i) 100 parts by weight of a polyamide polymer,0.001-10 parts by weight of a polyamide oligomer with a molecular weightof 5000 or less and having hydrocarbon radicals with 5-30 carbons as theterminals, and 1-25 parts by weight of a triazine flame retardant; ii)100 parts by weight of a polyamide-6, 6 polymer, 15 parts by weight of apolyamide oligomer with a molecular weight of 1000 and consisting of thecondensation product of stearic acid, ethylenediamine, and sebacic acid,and 7 parts by weight of melamine cyanuric acid; or iii) 100 parts byweight of a polyamide-6 polymer, 0.5 parts by weight of polyamideoligomer with a molecular weight of 1200 and consisting of thecondensation product of stearyl amine, ethylenediamine, and sebacicacid, and 27 parts by weight melamine cyanuric acid.
 2. Processaccording to claim 1, wherein the polyamide polymer is a polyamide witha melting temperature of at least 260° C.
 3. Process according to,wherein the polyamide oligomer is a polyamide with a melting temperatureof at least 260° C.
 4. Process according to claim 1, wherein the flameretardant is halogen-free flame retardant.
 5. Process according to claim1, wherein the flame retardant is a halogenated organic compound. 6.Process according to claim 1, wherein the polyamide compositioncomprises a reinforcing component.
 7. Flame retardant polyamide compoundcomprising a polyamide polymer having a weight-average molecular weightof at least 10,000 g/mol and a an amount of 1-100 wt. %, relative to thetotal weight of polyamide, of a flame retardant, characterized in thatthe compound comprises an amount of 0.1-30 wt. %, relative to the totalweight of polyamide, of a polyamide derived from a polyamide oligomerhaving a molecular weight of at most 7500 g/mol, with the proviso thatthe compound does not consist of: i) 100 parts by weight of a polyamidepolymer, 0.001-10 parts by weight of a polyamide oligomer with amolecular weight of 5000 or less and having hydrocarbon radicals with5-30 carbons as the terminals, and 1-25 parts by weight of a triazineflame retardant; ii) 100 parts by weight of a polyamide-6, 6 polymer, 15parts by weight of a polyamide oligomer with a molecular weight of 1000and consisting of the condensation product of stearic acid,ethylenediamine, and sebacic acid, and 7 parts by weight of melaminecyanuric acid; or iii) 100 parts by weight of a polyamide-6 polymer, 0.5parts by weight of polyamide oligomer with a molecular weight of 1200and consisting of the condensation product of stearyl amine,ethylenediamine, and sebacic acid, and 27 parts by weight melaminecyanuric acid.
 8. Use of a polyamide compound according to claim 8 forthe preparation of a molded part.